The precise monitoring and control of blood glucose levels has emerged as an esential ingredient in the development of long term therapy for diabetes. Although numerous attempts have been made to develop suitable in situ enzymatic glucose sensors, most suffer from serous limitations, including: poor measurement precision, non-linearity of signal output, interferences and signal drift. In order to minimize or eliminate these undesirable effects it will be necessary to understand the fundamental processes affecting electrode response. Using modern electrochemical techniques, it is proposed to examine and evaluate the rates of substrate and product transport to and from an amperometric sensor capable of detecting oxygen or hydrogen peroxide, substrate and product of the enzymatic reaction, respectively. The coupling of these phenomena to the enzymatic oxidation of glucose in a layer close to the sensor will be evaluated in the light of these other processes. The media supporting the enzyme and through which substrates, products and potential interferences must move will be optimized, taking into account the conditions of in vivo glucose monitoring. The performance of glucose sensors will be evaluated in undiluted whole blood and blood serum. The findings of these studies will be generally applicable to the development and optimization of enzyme based sensors for the monitoring of other species in biological fluids. The sensors will be tested under in vivo conditions in normal and diabetic rats.